1. Field of the Invention
The present invention relates to a control apparatus for controlling an AC rotary machine wherein the control apparatus can regulate the amount of induced voltage by means of an actuator.
2. Description of the Background Art
The amount of induced voltage produced in an AC rotary machine is proportional to the product of the magnitude of magnetic flux and rotating speed. In a case where a power converter is used to drive an AC rotary machine with variable rotating speed, there can arise such a situation that induced voltage produced in a high-speed range of the machine becomes so high as to exceed a maximum level of voltage which the power converter can output. In such kinds of AC rotary machine as an induction machine or a synchronous reluctance machine which does not use any permanent magnets, the magnitude of magnetic flux is proportional to a magnetic flux component of an armature current and, thus, it is possible to maintain the induced voltage within an output voltage range of a power converter by reducing the magnetic flux component of the armature current even when the AC rotary machine is driven in the high-speed range.
In an AC rotary machine using permanent magnets on the other hand, an induced voltage is produced due to the permanent magnets. Therefore, when the machine is operated in a high-speed range, it is necessary that an armature current contain a magnetic flux component for canceling magnetic flux produced by the permanent magnets, resulting in an increase in the armature current. This means that, when an AC rotary machine using permanent magnets is operated in a high-speed range, there arises a problem that the machine requires a power converter having a large current capacity or a problem related to loss or heat generation caused by an increase in the current.
Japanese Unexamined Patent Publication No. 2005-160278 proposes a synchronous motor implementing an arrangement intended for solving the aforementioned problems of the prior art. According to the Publication, the synchronous motor includes a stator having a plurality of magnetic teeth on which coils are wound and a rotor having permanent magnets, wherein electric power is supplied into the coils from the exterior to produce magnetic fields from the magnetic teeth so that the rotor generates a torque. The stator is dividend into at least two separate portions, at least one of the separate portions constituting a movable stator element which serves to vary phase of the magnetic teeth relative to the other portion of the stator. The synchronous motor is provided with a movable stator element control device for controlling phase of the movable stator element according to state of the movable stator element according to operating conditions of the synchronous motor.
This arrangement of the Publication makes it possible to reduce a counter electromotive force which occurs in the synchronous motor by varying the phase of the movable stator element so that the motor torque becomes zero when a zero torque command is given. Also, as the above arrangement eliminates the need for field-weakening current, it becomes possible to greatly reduce losses caused by drag rotation of the rotor. In addition, when rotating speed of the motor is equal to or above a threshold value, the phase of the movable stator element may be varied so that the counter electromotive force will not be produced. This arrangement of the Publication can increase the rotating speed of the motor without increasing an output voltage of an inverter or a battery.
A conventional control apparatus for controlling an AC rotary machine controls a phase difference between a stator and a movable stator element by means of an actuator as discussed above, the phase difference between the stator and the movable stator element being a control parameter of the actuator. Therefore, a control mechanism of the control apparatus requires means for detecting the phase difference between the stator and the movable stator element, the means being controlled by the actuator. The means required for this purpose is a position sensor which is generally complex and expensive. For this reason, there has been a problem that it has been impossible to employ such a low-cost actuator that can not detect the position of the movable stator element but can control only a time derivative value of the actuator phase difference.